Check if Rust lib needs rebuilding. (#13759)

This protects against the common mistake of failing to remember to rebuild Rust code after making changes.

1 files changed, 45 insertions, 0 deletions

diff --git a/rust/build.rs b/rust/build.rs
new file mode 100644
index 0000000000..2117975e56
--- /dev/null
+++ b/rust/build.rs
@@ -0,0 +1,45 @@
+//! This build script calculates the hash of all files in the `src/`
+//! directory and adds it as an environment variable during build time.
+//!
+//! This is used so that the python code can detect when the built native module
+//! does not match the source in-tree, helping to detect the case where the
+//! source has been updated but the library hasn't been rebuilt.
+
+use std::path::PathBuf;
+
+use blake2::{Blake2b512, Digest};
+
+fn main() -> Result<(), std::io::Error> {
+    let mut dirs = vec![PathBuf::from("src")];
+
+    let mut paths = Vec::new();
+    while let Some(path) = dirs.pop() {
+        let mut entries = std::fs::read_dir(path)?
+            .map(|res| res.map(|e| e.path()))
+            .collect::<Result<Vec<_>, std::io::Error>>()?;
+
+        entries.sort();
+
+        for entry in entries {
+            if entry.is_dir() {
+                dirs.push(entry)
+            } else {
+                paths.push(entry.to_str().expect("valid rust paths").to_string());
+            }
+        }
+    }
+
+    paths.sort();
+
+    let mut hasher = Blake2b512::new();
+
+    for path in paths {
+        let bytes = std::fs::read(path)?;
+        hasher.update(bytes);
+    }
+
+    let hex_digest = hex::encode(hasher.finalize());
+    println!("cargo:rustc-env=SYNAPSE_RUST_DIGEST={hex_digest}");
+
+    Ok(())
+}